Help: grep

Find changesets by keywords (author, files, the commit message), revision
number or hash, or revset expression.

hg grep [OPTION]... PATTERN [FILE]...

search for a pattern in specified files and revisions

Search revisions of files for a regular expression.

This command behaves differently than Unix grep. It only accepts
Python/Perl regexps. It searches repository history, not the
working directory. It always prints the revision number in which a
match appears.

By default, grep only prints output for the first revision of a
file in which it finds a match. To get it to print every revision
that contains a change in match status ("-" for a match that
becomes a non-match, or "+" for a non-match that becomes a match),
use the --all flag.